The immunoglobulin molecule consists of one or more sets of four polypeptide chains, two heavy chains molecular weight about 53,000 and two light chains molecular weight about 22,000, joined by disulfide bonds.
Five classes of immunoglobulin are distinguished by the presence of heavy-chain antigenic determinants, which are designated by the lower case Greek characters corresponding to the Roman letters applied to the immunoglobulins:
______________________________________ HEAVY-CHAIN IMMUNOGLOBULIN ANTIGENIC DETERMINANT ______________________________________ IgG .gamma. (gamma) IgA .alpha. (alpha) IgM .mu. (mu) IgD .delta. (delta) IgE .epsilon. (epsilon) ______________________________________
There are also subclasses of IgG, IgA, and IgM, based upon other antigenic determinants, which are designated by numerals (e.g., .gamma.1, .alpha.1, .mu.1). Four subclasses of IgG have been recognized, two of IgA, and two of IgM. All subclasses are found in the sera of all normal individuals.
IgG is the most abundant immunoglobulin in the serum of normal humans. It is also found in the tissue fluids, and it can cross the placenta from the maternal to the fetal circulations. It has antibacterial, antiviral, and antitoxic activities in vivo, and in vitro. It is a late responding antibody.
IgM is characterized by possession of heavy chains with the amino acid sequence that defines the antigenic determinant .mu.. A distinguishing feature of IgM function is its strong cytolytic and complement-fixing activity, which far exceeds that of IgG. IgM is usually the first antibody to appear in animals or humans following immunization. It is then gradually replaced by IgG.
IgA is the second most abundant immunoglobulin in human serum and is the chief secretory immunoglobulin. It is useful in antibacterial and respiratory viral defense.
IgE is the least abundant immunoglobulin. It has skin sensitizing properties and is responsible for a variety of bronchial, gastrointestinal, skin and other allergic reactions.
Very little is known about the structure and biological function of IgD. IgD antibodies have been found in patients sensitive to cow's milk and patients with systemic lupus erythematosus.
Methods for isolating the above immunoglobulin classes are well-known. Methods for raising antibodies to immunoglobulins are also known and methods for binding immunoglobulins to solid supports are known. Methods for isolating antigens, labeling antigen with fluorescent molecules, radioactive molecules or enzymes to permit measuring bound antigen are well-known. There are likewise, indirect methods for measuring bound antigen, such as reacting the bound antigen with a labeled (radioactive, fluorescent, enzyme) antibody to the antigen.
U.S. Pat. No. 4,020,151 describes an immunoassay for IgG, IgA, and IgM concentrations in the serum which comprises first reacting a solid support with test sample to adsorb IgG, IgA, IgM and then reacting a labeled antibody to IgG, IgA or IgM and measuring the bound antibody.
Bradley et al., Journal of Clinical Microbiology, May 1977, pages 521-530, describes the determination of IgG and IgM related to hepatitis A virus by radioimmunoassay. Antibody to hepatitis A virus in the test specimen was coated on a polyvinyl surface by reacting the polyvinyl surface with test serum. Purified hepatitis A antigen and .sup.125 I labeled immunoglobulin G (IgG), that is, anti-hepatitis A antibody, were subsequently reacted sequentially with the polyvinyl surface. Counting .sup.125 I bound to the well provided a determination of total anti-hepatitis A in test serum. Immunoglobulin M anti-hepatitis A was distinguished depending on whether the above anti-hepatitis A reactiion was inhibited by a goat anti-IgM. It is important to be able to distinguish antibodies to specific immunoglobulins since, for example, in hepatitis A infection, IgM antibodies are prevalent in the acute stage of the infection and IgG is prevalent in the convalescent stage. A large portion of the population have been exposed to hepatitis A and it is necessary to distinguish acute versus convalescent stage of infection, i.e., IgM versus IgG antibodies. The present invention provides a unique method for monitoring class specific antibodies during bacterial and viral infection as well as allergic reactions.